Flat panel displays have recently been developed for visually displaying information generated by computers and other electronic devices. These displays can be made lighter and require less power than conventional cathode ray tube displays. One type of flat panel display is known as a cold cathode field emission display a field emission display (FED).
A cold cathode FED uses electron emissions to illuminate a cathodoluminescent screen and generate an image. A single pixel 10 of a prior art FED is shown in FIG. 1A. The FED pixel 10 includes a substrate 11 formed with a conductive layer 12. An array of emitter sites 13 are formed on the conductive layer 12. Although each pixel 10 typically contains many emitter sites (e.g., 4-20 for a small display and several hundred for a large display), for simplicity only one emitter site 13 is shown in FIG. 1A. A grid 15 is associated with the emitter sites 13 and functions as a gate electrode. The grid 15 is electrically isolated from the conductive layer 12 by an insulating layer 18. The grid 15/conductive layer 12/substrate 11 subassembly is sometimes referred to as a baseplate.
Cavities 23 are formed in the insulating layer 18 and grid 15 for the emitter sites 13. The grid 15 and emitter sites 13 are in electrical communication with a power source 20. The power source 20 is adapted to bias the grid 15 to a positive potential with respect to the emitter sites 13. When a sufficient voltage differential is established between the emitter sites 13 and the grid 15, a Fowler-Nordheim electron emission is initiated from the emitter sites 13. The voltage differential for initiating electron emission is typically on the order of 20 volts or more.
Electrons 17 emitted at the emitter sites 13 collect on a cathodoluminescent display screen 16. The display screen 16 is separated from the grid 15 by an arrangement of electrically insulating spacers 22. The display screen 16 includes an external glass face 14, a transparent electrode 19 and a phosphor coating 21. Electrons impinging on the phosphor coating 21 cause the release of photons 25 which forms the image. The display screen 16 is the anode in this system and the emitter sites 13 are the cathode. The display screen 16 is biased by the power source 20 (or by a separate anode power source) to a positive potential with respect to the grid 15 and emitter sites 13. The potential at the display screen 16 is termed herein as anode. In some systems the potential at the display screen 16 is on the order of 1000 volts or more.
One method of addressing the emitter sites 13 for use in video displays is taught by Crost et al. in U.S. Pat. No. 3,500,102. In this method the emitter sites 13 are electrically connected and placed parallel to additional rows of emitter sites. The grids 15 associated with the emitter sites 13 are electrically connected in parallel columns which are orthogonal to the emitter rows. The emitter sites 13 associated with each pixel 10 of the FED are uniquely defined by the intersection point of a specific emitter row and a specific grid column. Electrically addressing a row while simultaneously addressing a column activates a specific pixel 10.
Another method for addressing the emitter sites 13 for use in video displays is disclosed by Casper et al. in U.S. Pat. No. 5,210,472. In this method, a common grid electrode is employed with respect to all of the pixels in the display. Addressing of the pixels within the display as taught by Casper et al. is accomplished with row and column electrodes which provide access for the emitter sites 13 to a source of electrons.
One problem in a FED that occurs during the turn on process (i.e., power up) is the emission of electrons from the emitter sites 13 to the grids 15. Emission to grid during turn on is illustrated in FIG. 1B. During the turn on process, electrons 26 emitted from the emitter sites 13 can go directly to the grid 15 rather than to the display screen 16. This situation can lead to overheating of the grids 15. The emission to grid can also affect the voltage differential between the emitter sites 13 and grids 15. In addition, desorped molecules and ions can be ejected from the grid 15 causing excessive wear of the emitter sites 13. Electron emission to grid can also lead to electrical arcing 30 between the grid 15 and the conductive layer 12 or emitter sites 13. In addition, electrons 26 emitted from the emitter sites 13 can strike the spacers 22 causing a charge build up on the spacers 22.
All of these problems decrease the lifetime, performance and reliability of the FED. Electron emission to grid is particularly a problem in consumer electronic products, such as camcorders, televisions and automotive displays, which are typically turned on and off many times throughout the useful lifetime of the product.
One reason for the electron emission to grid, is that electron emission may have commenced from the emitter sites 13 before the large voltage potential (V.sub.Anode) has been established at the display screen 16. Typically, the display screen 16 is a relatively large, relatively high voltage structure which requires some period of time to reach full potential across its entire surface. In addition, the display screen 16 operates at a significantly higher voltage than any other component of the FED. Some period of time is required to ramp up to this operating voltage. Consequently, the display screen 16 can be at a low enough positive potential to allow electron emission to grid 15 to occur, as illustrated in FIG. 1B. Although this situation may only occur for a relatively short period of time, it can cause system problems as outlined above.
A related situation can also occur during turn on of the display screen 16 and grid 15 if the emitter sites 13 are not electrically controlled. If the emitter sites 13 are not limited during power on, an uncontrolled amount of emission can occur causing the same problems as outlined above.
In addition, a similar situation exists during the turn off process for the FED cell 10 (i.e., power off). If power to the large positive potential at the display screen 16 is lost prior to termination of electron emission from the emitter sites 13, then electron emission to grid, as illustrated in FIG. 1B, can occur.
In view of these problems associated with field emission displays, it is an object of the present invention to provide an improved method for controlling field emission displays to prevent electron emission to grid during turn on and turn off. It is yet another object of the present invention to provide an improved control circuit for an FED adapted to reduce electron emission to grid during turn-on and turn off. Other objects, advantages and capabilities of the present invention will become more apparent as the description proceeds.